1. Field of the Invention
The present invention relates to a projection optical system for obliquely projecting a two-dimensional image formed by scanning light from a light source and an optical system suitable for oblique projection or oblique image pickup.
2. Description of the Related Art
As a projection optical system used for a forward projection type projector, a projection optical system capable of obliquely projecting an image on a screen to shorten the distance between the screen and an apparatus has been proposed.
FIG. 13 shows a projector disclosed in Japanese Patent Application Laid-Open No. 5-100312. Referring to FIG. 13, this projector includes an illumination light source L and a light valve LV using a reflection dot matrix liquid crystal device or the like. A projection optical system enlarges and projects an image formed on the light valve LV onto a screen S. In this projector, a wide-angle lens having a large field angle is used as a projection optical system, the light valve LV and screen S are so arranged to be shifted with respect to the optical axis of the projection optical system, and projection is performed by using an end portion of the field angle, thereby forming an oblique projection optical system.
FIG. 14 shows the projector disclosed in Japanese Patent Application Laid-Open No. 5-080418. Referring to FIG. 14, this projector includes an illumination light source L and a light valve LV using a transmission or reflection dot matrix liquid crystal device or the like. The first projection optical system forms an intermediate image of an image formed on the light valve LV. The second projection optical system enlarges and projects this intermediate image on a screen S. In this projector, the first and second projection optical systems are inclined with respect to the optical axis to perform oblique projection on the screen S.
In addition, International Publication No. WO97/01787 discloses a projection optical system constituted by a plurality of reflecting surfaces.
Japanese Patent Application Laid-Open No. 9-5650 discloses a design method for a decentered optical system and a method of calculating a paraxial quantity such as a focal length. In addition, Japanese Patent Application Laid-Open Nos. 8-292371, 8-292372, and 9-222561 disclose design methods for such optical systems. It has become clear from these design examples that an optical system whose aberration is sufficiently corrected can be formed by introducing the concept of a reference axis and forming a constituent surface using an asymmetrical spherical surface.
Such a decentered optical system is called an off-axial optical system. Considering a reference axis extending along a ray passing through an image center and pupil center, this optical system is defined as an optical system including a curved surface (off-axial curved surface) which is designed such that the normal to the constituent surface at the intersection between the surface and the reference axis does not exist on the reference axis. The reference axis has a bent shape.
In this off-axial optical system, the constituent surface becomes decentered, and no vignetting occurs on the reflecting surfaces, thus facilitating the construction of an optical system using reflecting surfaces. In addition, an intermediate image is formed in the optical system. This makes it possible to form a compact optical system with a large field angle.
Although this optical system is a front stop optical system, optical paths can be routed relatively freely. This makes it possible to form a compact optical system.
In addition, Japanese Patent Application Laid-Open No. 6-295159 discloses an apparatus which displays an image by two-dimensionally scanning a laser beam with a rotating polyhedral mirror. In this case, distortion can be corrected by setting the timing of intensity modulation of a laser beam to a proper value, i.e., electrical correction.
As disclosed in Japanese Patent Application Laid-Open No. 5-100312, in the projection optical system in which the light valve and screen are shifted from the optical axis, although the field angle to be used is θ2, a large lens system with a large field angle which covers a considerably large field angle (θ1) is required, as shown in FIG. 15.
When an optical axis A extends to the center of the screen S as shown in FIG. 16, an image is not formed on the screen S in a general lens system but is formed on a plane S′ perpendicular to the optical axis. In this case, the image undergoes trapezoidal distortion. As a result, the image falls out of focus on the screen S in the vertical direction.
In order to correct the inclination of this image plane, the difference between an optical path L1 of a ray passing through an upper portion of the screen S and an optical path L2 of a ray passing through a lower portion of the screen S must be canceled out. In correcting this different, if correction can be made near the imaging plane, since the difference between the optical paths L1 and L2 is reduced, the correction amount can be reduced. If, however, correction is made on an optical pane on the screen side where the image is enlarged, the difference between the optical paths L1 and L2 directly affects the correction.
In the optical system disclosed in Japanese Patent Application Laid-Open No. 5-80418, since the lens system is simply tilted, it is difficult to incline an image plane sufficiently. In addition, if the tilt amount of the lens system is excessively large, it is difficult to ensure high optical performance.
In the optical system disclosed in International Publication No. WO97/01787, one reflecting surface increases in size, resulting in difficulty in manufacturing the optical system with high precision.
As disclosed in Japanese Patent Application Laid-Open No. 6-295159, in the method of displaying an image by scanning a point light source or one-dimensional image display element with a rotating polyhedral mirror or galvano mirror, distortion may not satisfactorily be corrected by electrical correction alone. In this case, optical correction is required.